1. Field of the Invention
This invention relates to a predistortion equalizing circuit for cancellation of second order distortion caused by a serially connected light emitting diode. Accordingly, it is a general object of this invention to provide new and improved circuits of such character.
2. Description of the Prior Art
A light emitting diode, typically, tends to distort an incoming signal applied thereto. That is, an output optical signal from the light emitting diode is not a linear representation of the electrical signal applied thereto.
In the past, others have endeavored to create a circuit that would linearize a light emitting diode to reduce interfering distortion of an analog signal applied thereto. Predistortion, feed forward, and optical feedback circuits have been used to linearize the outputs of light emitting diodes. Predistortion, as used in the past, utilized diode breakpoints to create an opposite transfer characteristic, though it cancelled the non-linearity of a light emitting diode characteristic curve. Feed forward utilizes two identical light emitting diodes in which the applied signal is split in two: one signal driving one light emitting diode to create a non-linearity which is then detected and subtracted from the drive signal to the second light emitting diode to cancel the non-linearity of the second. Optical feedback takes a sample of the light output from the light emitting diode, detects it, and subtracts it from the incoming signal, very much like a standard feedback amplifier, thus linearizing the light emitting diode.
These techniques of the prior art have not been very successful. Disadvantageously, the prior art predistortion circuits required large voltage swings and many adjustable breakpoints to create the correct predistortion. Large voltage swings required high impedances which were frequency limited by circuit capacitance. Adjustments of the breakpoints were quite tedious when the distortion components required reduction to 50 to 60 dB below the input signal.
Feed forward required an additional optical receiver as well as an identical light emitting diode. Any subtle differences between the light emitting diodes and the driver circuits hindered cancellation of the nonlinearity and could create new forms of distortion.
Optical feedback was limited by phase lag and time delay through the driver and feedback loop. That, in turn, limited the practical use of such a technique to below the required bandwidth for multi-channel service (10 MHz). The expense of an additional receiver and optical tap was also a drawback.